P1048: MYF3001: A RANDOMIZED OPEN LABEL, PHASE 3 STUDY TO EVALUATE IMETELSTAT VERSUS BEST AVAILABLE THERAPY IN PATIENTS WITH INTERMEDIATE-2 OR HIGH-RISK MYELOFIBROSIS REFRACTORY TO JANUS KINASE INHIBITOR
Authors: J. Mascarenhas, C. N. Harrison, J.-J. Kiladjian, R. S. Komrokji, S. Koschmieder, A. M. Vannucchi, T. Berry, D. Redding, L. Sherman, S. Dougherty, L. Peng, L. Sun, F. Huang, Y. Wan, F. M. Feller, A. Rizo, S. Verstovsek
Journal: HemaSphere (2022)

Background: Myelofibrosis (MF) is a life-threatening myeloproliferative neoplasm. The Janus Kinase inhibitors (JAKi) ruxolitinib and fedratinib are the only FDA approved treatment options for MF. Despite benefits reported with ruxolitinib in the front-line setting, a high proportion of patients discontinue treatment (Abdelrahman 2015), and the median overall survival (OS) is 11-16 months (Kuykendall 2018; Newberry 2017; Schain 2019; Palandri 2019; Mascarenhas 2020), highlighting a great unmet need for pts non-responsive to a JAKi treatment. Imetelstat, a first-in-class telomerase inhibitor, has shown meaningful clinical improvement in IMbark, a Phase 2 study in patients with intermediate-2 or high-risk MF who have relapsed after or are refractory to JAKi (Mascarenhas JCO 2021). Treatment with 9.4 mg/kg imetelstat resulted in 32.2% symptom response (total symptom score [TSS] reduction ≥50%) at Week 24 and median overall survival (OS) of 29.9 months with overall study follow up of 27.4 months. Dose-dependent inhibition of telomerase with imetelstat resulted in on-target activity that correlated with clinical benefits; dose-dependent reduction in variant allele frequency of MF driver mutations indicated targeting of the underlying malignant clone. Aims: The Phase 2 results support continued study of imetelstat 9.4 mg/kg in a Phase 3 randomized controlled study, for which the study design is described here. Methods: Study MYF3001 (IMpactMF; NCT04576156) is an open label, randomized (2:1), multicenter, Phase 3 study of imetelstat compared with best available therapy (BAT) in ~320 patients with intermediate 2 or high-risk MF refractory to JAKi treatment. Patients will be randomized to receive imetelstat 9.4 mg/kg IV every 21 days or investigator selected BAT including hydroxyurea, thalidomide, interferon, danazol, hypomethylating agents, chemotherapy, or other non-JAKi containing therapy as appropriate). Eligible patients will be stratified based on a) Intermediate 2 or high-risk per Dynamic International Prognostic Scoring System; b) platelet count at entry (platelets ≥ 75 and < 150 x 109/L vs ≥ 150 x 109/L). Patients who meet progressive disease criteria and discontinue BAT may be eligible to crossover to imetelstat. Results: The primary endpoint is OS and one interim analysis is planned when approximately >71% of death events have occurred. Secondary endpoints include symptom and spleen response rates at week 24, progression-free survival, clinical response assessment per modified 2013 International Working Group - Myeloproliferative Neoplasms Research and Treatment criteria, time to and duration of response, reduction in degree of bone marrow fibrosis, safety, pharmacokinetics and patient-reported outcomes. Biomarkers and mutation analyses will be performed to evaluate the impact of imetelstat on reduction/depletion of malignant clones. (Figure 1) Image:Summary/Conclusion: Approximately 180 sites are planned in North and South America, Europe, Middle East, Australia and Asia. The study is open for enrollment.